In mobile communication, compression coding for digital information of speech and images is essential for efficient use of transmission band. Here, expectations for speech codec (coding and decoding) techniques widely used in mobile telephones are high, and further sound quality improvement is in demand in addition to conventional high-efficiency coding of high compression performance. Further, speech communication is a basic function of mobile telephones and therefore is essential to be standardized, and, given the tremendous value of intellectual property rights it entails, is actively researched and developed by companies all over the world.
The basic scheme “CELP (Code Excited Linear Prediction),” which models the vocal system of speech established about twenty yeas ago and which adopts vector quantization skillfully, has improved decoded speech quality significantly. Further, the emergence of techniques using fixed excitations comprised of a small number of pulses like with an algebraic codebook (e.g., disclosed in Non-Patent Document 1) has marked further advancement in speech coding performance.
However, in CELP, as for spectrum envelope information, high efficiency coding methods such as line spectrum pair (“LSP”) parameters and prediction VQ (Vector Quantization) are developed, and, as for a fixed codebook, high efficiency coding methods are developed such as the above-noted algebraic codebook. However, few studies have been made to improve performance of only an adaptive codebook.
Therefore, although sound improvement of CELP has peaked up till now, to solve this problem, Patent Document 1 discloses a technique of limiting a frequency band of adaptive codebook code vectors (hereinafter “adaptive excitations”) by the filter adapted to an input acoustic signal and using the code vectors after the frequency band limitation to generate synthesis signals.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-29798    Non-Patent Document 1: Salami, Laflamme, Adoul, “8 kbit/s ACELP Coding of Speech with 10 ms Speech-Frame: a Candidate for CCITT Standardization”, IEEE Proc. ICASSP94, pp. II-97n